Production of high-purity niobium monoxide and capacitor production therefrom

ABSTRACT

The present invention relates to high-purity niobium monoxide powder (NbO) produced by a process of combining a mixture of higher niobium oxides and niobium metal powder or granules; heating and reacting the compacted mixture under controlled atmosphere to achieve temperatures greater than about 1800° C., at which temperature the NbO is liquid; solidifying the liquid NbO to form a body of material; and fragmenting the body to form NbO particles suitable for application as e.g., capacitor anodes. The NbO product is unusually pure in composition and crystallography, highly dense, and can be used for capacitors and for other electronic applications. The method of production of the NbO is robust, does not require high-purity feedstock, and can reclaim value from waste streams associated with the processing of NbO electronic components.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/428,430, filed May 2, 2003.

FIELD OF THE INVENTION

The present invention relates to methods of producing niobium monoxidepowders of high purity, and the use of such niobium monoxide powders inthe production of valve devices, i.e., capacitors.

BACKGROUND OF THE INVENTION

It has been long recognized that niobium monoxide (NbO) has some unusualelectrical properties that make it well-suited for the manufacture ofelectronic capacitors. For example, it is of much lower flammabilitythan equivalent tantalum powders, is less costly than tantalum, and hasa much larger potential supply than tantalum. However, niobium monoxidecapacitor powders require high levels of purity, with not only foreignelements such as iron and copper being deleterious, but other forms ofniobium such as niobium metal, niobium dioxide (NbO₂), niobium trioxide(Nb₂O₃) and niobium pentoxide (Nb₂O₅) being potentially harmful as well.In order to be useful in a valve application, the niobium monoxideshould be in a finely divided form, i.e., a fine powder or agglomeratesformed from small particles, typically 1-2 microns in diameter, orfiner. In order to meet these prerequisites, the electronics industryhas produced niobium monoxide by reacting niobium pentoxide or niobiumdioxide (possibly pre-reduced from the pentoxide) with a metallicreducing agent under conditions in which the niobium oxides remain inthe solid state. This allows the particle morphology of the originaloxide to be preserved in the niobium monoxide.

In one embodiment of this process, niobium pentoxide is reacted attemperatures of approximately 1000° C. with finely divided metallicniobium in such stoichiometric proportions as to produce primarilyniobium monoxide. In another embodiment, the niobium pentoxide orniobium dioxide is reacted with gaseous magnesium, similarly attemperatures of approximately 1000° C. This results in a “spongy”niobium monoxide-magnesium oxide mixture. After leaching the magnesiumoxide, the resultant product is a high-surface area, agglomerated massof niobium monoxide.

Because of the low processing temperatures used in these methods ofproducing niobium monoxide, there is inadequate opportunity to removeimpurities in either the niobium oxide or the reducing agent feedstock.The purity requirements of the niobium monoxide dictate the purityrequired of the feedstock. The surface area requirements of the productniobium monoxide further dictate the particle size distribution andmorphology of the niobium pent-or-dioxide required for the process.These requirements severely limit the availability of suitable rawmaterials. In addition, because the reactions occur in the solid state,the reactions are sluggish, and often do not go to completion. Theproduct contains some higher oxides of niobium, and often some niobiummetal.

Thus, an object of the present invention is to produce niobium monoxide(NbO) powder of high purity and sufficient surface area to meet therequirements of NbO capacitors without the constraints of raw materialspurity and particle size imposed by solid-state processes, and furtherto the use of such powders in the production of capacitors.

SUMMARY OF THE INVENTION

The present invention relates to a high-purity niobium monoxide powder,produced by a process comprising:

(a) combining a mixture of niobium pentoxide, niobium trioxide and/orniobium dioxide and coarse niobium metal powder in effective amountsstoichiometrically calculated to yield a product with a fixed atomicratio of niobium to oxygen, the ratio being preferably close to 1:1;

(b) forming a compact of the mixture by cold isostatic pressing or otherappropriate techniques;

(c) exposing the compact to a heat source sufficient to elevate thesurface temperature above the melting point of the product niobiummonoxide, i.e., greater than about 1800° C. in an atmosphere suitable toprevent uncontrolled oxidation;

(d) allowing the mixture to react exothermically to produce the desiredniobium monoxide;

(e) solidifying the mixture to form a solid body of niobium monoxide;and

(f) fragmenting the body to form the desired particle size of niobiummonoxide.

Capacitor anodes can thereby be produced from niobium oxide particles,by techniques common to the capacitor industry.

In preferred embodiments, the weight ratio of Nb₂O₅ to metallic niobiumin the mixture is about 1:1; the weight ratio of NbO₂ to metallicniobium in the mixture is about 1.3:1; and the weight ratio of Nb₂O₃ tometallic niobium in the mixture is about 2.5:1. The heat source ispreferably an electron beam furnace, a plasma-arc furnace, an inductionfurnace, or an electric resistance furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of theinvention as well as other information pertinent to the disclosure, inwhich:

FIGS. 1 a-c are graphs of x-ray diffraction patterns for NbO produced bythe present invention (FIGS. 1 a-b), and NbO produced by a commercial,solid-state reaction (FIG. 1 c).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method of producing niobium monoxidepowder, which includes combining a mixture of Nb₂O₅, Nb₂O₃ and/or NbO₂,and niobium metal, forming a compacted bar of the mixture; reacting themixture at a temperature greater than about 1800 C; solidifying thereaction products; and fragmenting the solidified body to form niobiummonoxide powder. In a preferred embodiment of the present invention, theweight ratio of niobium pentoxide to niobium metal is about 1:1.

The present invention also relates to the production of a high-purityniobium monoxide powder produced by this process from impure niobiumpentoxide and/or impure niobium dioxide, and from impure niobium metalpowder. In the present invention, the high processing temperature,controlled atmosphere and presence of a liquid state may be exploited toremove major impurities, including iron, aluminum, and various otherelements other than oxygen and refractory metals.

In the testing of the present invention, a mixture of commerciallyavailable, 99.99% pure Nb₂O₅ and commercially available, electron-beamtriple-refined dehydrided niobium metal powder (50×80 US mesh) wasblended and formed into a bar by cold isostatic pressing, although othermeans of compaction and resultant physical forms would also beeffective. Three such bars were prepared.

The compacts of Nb₂O₅ and niobium metal (weight ratio 1:1) were each fedsequentially into the melting region of an electron beam vacuum furnace,where each compact reacted and liquefied when heated by the electronbeam, with the liquid product dripping into a cylindrical, water-cooledcopper mold. When the electron beam initially struck the compact,melting immediately took place, with only a small increase in chamberpressure. A production rate of 100 pounds an hour was established.Reaction was terminated before the final compact had been fullyconsumed, leaving a layer of partially-reacted materials on the face ofthe residual compact.

While an electron-beam furnace was used in this experiment, it isanticipated that other energy sources capable of heating the materialsto at least 1800° C. could also be used, including, but not limited to,cold crucible vacuum induction melting, plasma inert gas melting, andelectrical impulse resistance heating.

The resultant ingot was allowed to cool under vacuum, and the apparatuswas vented to atmosphere. Samples were taken from the top one inch ofthe ingot (the “top” samples), while “edge” samples were taken fromlower mid-radius locations in the ingot.

Subsequent analysis of the product NbO samples by x-ray diffractionshowed a “clean” pattern for NbO, with no additional lines attributableto niobium metal, NbO₂ or Nb₂O₃. In FIG. 1 the x-ray diffractionpatterns are shown for NbO produced by the present invention, (edgesample in FIG. 1 a, top sample in FIG. 1 b) and NbO produced by acommercial, solid-state reaction (FIG. 1 c). The solid-state reactionproduct had numerous lines not originating with NbO, indicating thepresence of other, undesirable phases.

The ingot was then degraded to powder by conventional crushing, grindingand milling techniques. The resultant NbO powder had a Microtrac D50 of2.38 microns and a B.E.T. surface area of 2.06 m²/gram. When formed intoa capacitor anode under conventional conditions (Forming Voltage 35 V;Forming current 150 mA/g, sintered at 1400° C.), the anodes showedspecific capacitance at a 2 volt bias of 60,337 CV/g and a DC Leakage of0.31 nA/CV. Tested with a 0 volt bias, the specific capacitance was78,258 CV/g and the DC Leakage was 0.23 nA/CV. All of these values arewell within the normal range for commercial capacitors produced from NbOmade by solid-state reactions, as well as some tantalum capacitors.

Four additional experimental runs were performed using less purefeedstock and altering the sizing of the feedstock used to make thecompacts. In each run, the product was NbO free of other compounds andfree of metallic niobium. This indicated that the subject process wasrobust and not dependent on particular sources of oxides or niobiummetal. In one experimental run, the niobium pentoxide used as feedstockcontained approximately 400 ppm of iron, and the niobium metal containedless than 50 ppm of iron. After converting the feedstock to NbO by theprocess of the present invention, the NbO was analyzed and found tocontain less than 100 ppm of iron. This indicated a reduction of atleast 50% in the iron content during the process.

The NbO ingot from each of these four additional experimental runs wasreduced in size by conventional crushing, grinding and milling to anaverage particle size under 2.5 microns, formed into test anodes, andtested for capacitance and leakage rates. The results in each case weresimilar to the initial results described above, including anodesproduced from NbO originating from the high-iron feedstock.

The process of the present invention also serves to recover NbO valuesfrom waste streams associated with production of powder-based NbOproducts, since the refining action of the present invention caneffectively remove most contaminants, even when such contaminants arepresent as fine or micro-fine powders or particles.

The formation of niobium monoxide by melt phase processing lends itselfto the recovery and remelting of niobium monoxide solids, including butnot limited to powders, chips, solids, swarf (fine metallic filings orshavings) and sludges. Off-grade powder, recycled capacitors and powderproduction waste are among the materials that can be reverted to fullvalue niobium monoxide by this process.

While the present invention has been described with respect toparticular embodiments thereof, it is apparent that numerous other formsand modifications of the invention will be obvious to those skilled inthe art. The appended claims and this invention generally should beconstrued to cover all such obvious forms and modifications, which arewithin the true spirit and scope of the present invention.

1. A high-purity niobium monoxide (NbO) powder, produced by a processcomprising: a) combining a mixture of (1) a niobium oxide selected fromthe group consisting of Nb₂O₅, NbO₂ and Nb₂O₃, and (2) metallic niobium,wherein the niobium oxide and metallic niobium are present in powder orgranular form; b) forming a compact of the mixture; c) reacting themixture with a heat source, so that a mixture temperature greater thanabout 1800° C. is reached; d) solidifying the reacted mixture to form abody of material; and e) fragmenting the body of material to form theNbO powder.
 2. The niobium monoxide powder as recited in claim 1,wherein the weight ratio of Nb₂O₅ to metallic niobium in the mixture isabout 1:1.
 3. The niobium monoxide powder as recited in claim 1, whereinthe weight ratio of NbO₂ to metallic niobium in the mixture is about1.3:1.
 4. The niobium monoxide powder as recited in claim 1, wherein theweight ratio of Nb₂O₃ to metallic niobium in the mixture is about 2.5:1.5. The niobium monoxide as recited in claim 1, wherein the niobium oxideis Nb₂O₅.
 6. The niobium monoxide as recited in claim 1, wherein theheat source is an electron beam furnace.
 7. The niobium monoxide asrecited in claim 1, wherein the heat source is a plasma-arc furnace. 8.The niobium monoxide as recited in claim 1, wherein the heat source isan induction furnace.
 9. The niobium monoxide as recited in claim 1,wherein the heat source is an electric resistance furnace.
 10. Theniobium monoxide as recited in claim 1, wherein electronic valves areproduced from the niobium monoxide powders.